In code division multiple access (CDMA) networks, the mobile stations share a reverse link channel and may transmit simultaneously on the reverse link channel. During transmission, each mobile station spreads its transmitted signal with a spreading code selected from a set of mutually orthogonal spreading codes. The base station is able to separate the signals received from the mobile stations by a correlation process. For example, if the base station desires to receive the signal transmitted by mobile station A, the base station correlates the received signal with the spreading code used by mobile station A to despread the signal from mobile station A. All other signals will appear as noise due to lack of correlation. The base station can despread signals from all other mobile stations in the same manner.
CDMA networks are interference-limited systems. Since all mobile stations operate at the same frequency, internal interference generated within the network plays a critical role in determining system capacity and signal quality. The transmit power from each mobile station contributes to the noise floor and needs to be controlled to limit interference while maintaining desired performance objectives, e.g., bit error rate (BER), frame error rate (FER), capacity, dropped-call rate, coverage, etc. If the noise floor is allowed to get too high, widespread outages may occur. An outage is considered to occur when the power required to maintain minimum signal quality standards is greater than the maximum transmit power of the mobile station.
Rate control is one technique used to control the transmit power of a mobile station in a CDMA network. In general, the power required to maintain a desired signal quality increases as the data rate for transmission increases, and decreases as the data rate for transmission decreases. When a mobile station is commanded to transmit at a given data rate, the mobile station will transmit at the minimum power level needed to maintain acceptable signal quality standards. Thus, one way of controlling the transmit power of a mobile station is to dynamically adjust the data transmission rate of the mobile stations depending on reverse link load.
One rate control technique is known as common rate control. With common rate control, all mobile stations that need to transmit data in the reverse link are allowed to do so. Each mobile station initially begins transmitting at a specified minimum rate (sometimes called the autonomous rate) and then, depending on load of the base stations in its active set, is allowed to vary its transmission rate. The base stations periodically estimate the reverse link load and compare the estimated reverse link load to a target load. If the load is below a target threshold, a base station commands the mobile stations in its cell to increase their transmission rate. Conversely, if the load is above the target threshold, a base station commands the mobile stations in their respective cells to decrease their transmission rate. In some cases, the base station may command the mobile stations to hold their current transmission rate.
With common rate control, a base station broadcasts a single up/down/hold rate control command to all mobile stations in a cell or sector and all of the mobile stations respond to the extent that they are able. That is, when a base station commands the mobile stations in a cell to increase their transmission rate, all mobile stations in the cell except those already transmitting at maximum power will increase their transmission rate. When a base station commands the mobile stations in a cell to decrease their transmission rate, all mobile stations except those already transmitting at minimum power will decrease their transmission rate. Thus, common rate control results in significant fluctuations in load at the base station. These fluctuations are taken into account when setting the target load. Increasing the target load, in general, will increase system throughput at the risk of more frequent outages. Decreasing the target load will reduce outages at the cost of decreased throughput. The target load is therefore selected to balance system throughput against the probability of outages.